1. Field of the Invention
The present invention relates to a communication system and a communication method for the same and a mobile station and a base station used for the same, and more particularly, to the improvement of a communication channel quality informing system for informing a downlink communication channel quality of a plurality of different frequency-bands in downlink packet communication using a shared channel in a mobile communication system.
2. Description of the Prior Art
As a packet communication system for performing a communication as a plurality of mobile stations share a radio band, a HSDPA (High Speed Downlink Packet Access) or a LTE (Long Term Evolution) that are standardized with the 3GPP are known. In such a packet communication system, the mobile station measures the receiving quality of a pilot channel transmitted in the downlink and informs the measurement in the uplink as a communication channel quality (CQI: Channel Quality Indicator).
Correspondence between the receiving quality and the CQI is decided in advance, for example, CQIs by the 30 levels (information in five bits) are defined in the HSDPA. The radio base station uses a CQI informed from the mobile station for packet scheduling for performing allocation of transmission opportunities, and for adaptive modulation for changing a modulating method and a coding rate for the communication channel coding. By using the communication channel quality in such a manner, a communication can be provided according to the communication channel for each mobile station; thus, a radio band can be efficiently used.
For the access system in the downlink of the LTE, the OFDM is considered. The radio band allocated to the system is divided into a plurality of small radio bands (RB: Resource Block) so that the mobile station informs a CQI for each RB to the radio base station. As a mobile station is allocated to each RB, the frequency division multiplex is realized. For example, relationship between the system band and the RB is shown in FIG. 54. Here, K represents the total number of RBs.
Related documents include 3GPP TSG RAN, TR25.814 ver. 1.2.0, “Physical Layer Aspects for Evolved UTRA”, February 2006.
As a wireless band allocated to the system is getting wider, however, the transmission rate in the downlink increases and also the number of RBs increases. For example, in the LTE, the band of the RB is around 375 kHz, and in the system band of 5 MHz, it is divided into twelve. The band of the RB depends little on the width of the system band. Thus, as the system is getting to have a wider band, the number of CQIs for the mobile station needs to inform increases.
The mobile station uses the uplink to inform the CQI. In the LTE, however, the uplink is also a shared channel and uses the frequency division multiplex, thus, the number of the mobile stations that can transmit at a time is limited. Specifically, as the number of mobile stations to connect with the base station increases, a cyclical transmitting method is also used for increasing the number of multiplexing in the uplink.
On the other hand, as the CQI is used for scheduling or adaptive modulation in the downlink, if the CQI cannot inform the communication channel quality in an appropriate cycle, no scheduling nor adaptive modulation adapted to the communication channel quality of the mobile station is realized. Thus, it lowers a throughput in the downlink.
An object of the present invention is to provide a method for informing a communication quality which can transmit CQI required for scheduling or adaptive modulation in short cycles even if the number of the mobile stations to be connected with increases by reducing the amount of transmission for a time of transmission.